Packing tools are used to selectively isolate sections of wellbores; typically the packing tools are mounted on a mandrel lowered on production tubing or the like into a bore. The packing tool includes a resilient element which normally allows fluid to flow between the tool and the lining of the bore. Actuation of the tool deforms and expands the resilient element such that it contacts the bore lining; this prevents fluid flow beyond the packing tool location effectively isolating a section of the wellbore.
Packing tools are conventionally of two types, distinguished by the method of actuation of the tool. Inflatable packers are, as the name suggests, actuated by allowing fluid pressure to increase behind a section of the packing element thereby inflating the packer into contact with the liner. Production or test packers compress the resilient packing element between two plates or the like, causing the packing element to bow outwardly into contact with the bore lining.
A disadvantage of conventional production packing tools is that a high axial force is generally needed to provide the necessary deformation and expansion of the packing element, and to maintain it in the expanded position. Inflatable packing tools, on the other hand, rely on maintaining sufficient fluid pressure to maintain the seal; in the event of a pressure drop, the seal may fail.
U.S. Pat. No. 5,467,822 (Zwart) describes a packing tool which combines aspects of inflatable and production packing tools, by providing a resilient packing element between two compression rings, with the packing element including a fluid communication channel extending from an outer surface to an inner cavity. The packing element is expanded by compression from the compression rings, while well fluid enters the communication channel and provides an additional expansion force to the packing element. However, for this tool to function, it is necessary for well fluid to pass between the element and the wellbore liner over at least a portion of the packing element to reach the fluid communication channel. This arrangement leads to an increased risk that well fluid may penetrate the seal between the packing element and the bore liner, leading to a failure of the seal.
It is among the objects of embodiments of the present invention to obviate or alleviate these and other disadvantages of conventional packing tools.